The invention concerns a method for driving a power supply of a magnetic field coil for generating a predetermined magnetic field profile B(r,t) in the volume under investigation of a nuclear magnetic resonance (=NMR) apparatus, wherein an input signal i(t) that predetermines the time behavior of the magnetic field profile, is previously distorted to compensate for distortions caused by the apparatus, and wherein the power supply is driven by the pre-distorted signal o(t).
A method of this type is disclosed e.g. in WO 99/50681.
Nuclear magnetic resonance (NMR) is an important method of instrumental analytics and imaging diagnostics. NMR evaluates the reaction of nuclear spins, which are disposed in a strong magnetic field, to electromagnetic pulses. In particular, in imaging NMR (also called NMR tomography), magnetic field gradients are applied in a volume under investigation in order to obtain encoding, in particular, spatial encoding of the nuclear spins in the volume under investigation. The magnetic field gradients are generated by gradient coils.
During typical NMR measuring sequences, the magnetic field gradients must be switched on and off. A control current is switched for switching a magnetic field gradient. This control current is amplified in a power supply, and the amplified current of the power supply drives an associated gradient coil. The magnetic field generated by the gradient coil does not directly follow the control current flow. Due to eddy currents which are induced in conducting parts of the gradient coil itself and in its surroundings during switching, the magnetic field is distorted directly following current switching. The amplification characteristics of the power supply can also cause field distortions. After e.g. stepped switching of the current, the gradient field only changes slowly and mostly approximately exponentially from its previous value to the new desired value (equilibrium value, desired magnetic field profile). This deviation of the gradient field from a desired value deteriorates the quality of the NMR measurement.
In prior art, distortion of the gradient field is counteracted by pre-distorting the current of the gradient coil. The high-frequency portions of the control current are thereby excessively increased by subjecting a copy of the control current signal to high-pass filtering and adding it to the control current signal. The high-pass filtered copy may either be generated with an analog circuit through suitable inductances and capacitances, or digitally through simulation of high passes (see WO 99/50681).
Iterative methods are conventionally used in order to optimize the current profile in the gradient coil. The distortion circuit mostly comprises several parallel high passes of first order with different time constants. Iteration optimizes the portion of the individual high passes of the distorted current signal.
The gradient field approaches the desired value through conventional measures. An improved coil design additionally reduces the influence of eddy currents on the gradient field. However, the switching speeds of the gradient coils have recently increased, which, in turn, has deteriorated the problems of eddy currents and distortion of the gradient field.
The conventional methods of distortion compensation can only partially compensate for the existing distortions of the gradient field. In particular, they cannot compensate for temporarily oscillating distorted portions. Correlations between several magnetic field coils are also mostly ignored and not compensated for.
It is therefore the underlying purpose of the present invention to further improve compensation of distortions of the gradient field during fast switching and, in particular, permit compensation of oscillating distorted portions and optionally of correlations among the magnetic field coils.